NMR relaxation times for soil texture estimation in the laboratory: A comparison to the laser diffraction and sieve–pipette methods

Abstract

AbstractNuclear magnetic resonance (NMR) relaxation time (T2) distributions are well known to be linked to the pore size distribution and show promise as a method of estimating soil texture. As traditional laboratory methods used for soil texture estimates in soil science are generally time consuming, in this study, we explore an alternative approach based on NMR T2 distributions to estimate the soil texture of water‐saturated soil samples collected from three field sites. Using two T2 cut‐off times, T2a and T2b, the T2 distribution of a soil was partitioned into three regions, short, intermediate and long relaxation times, each of which represents the fraction of clay, silt and sand, respectively. Two approaches for determining the cut‐off times were used: the first used T2 cut‐off times determined from the data from all sites and the second used site‐specific T2 cut‐off times. The NMR estimates of soil texture were compared to measurements of soil texture made using the sieve–pipette method and laser diffraction particle size analysis (LDPSA). The results show that there is no universal cut‐off time for estimating the clay, silt and sand fractions based on the NMR T2 distributions. The accuracy of NMR measurements to estimate the soil texture depends on the magnetic susceptibility of the measured material. For soils with low magnetic susceptibility (<2 × 10−4 SI) using site‐specific cut‐off times, the NMR‐derived soil texture (root mean squared error [RMSE] = 9.43%) more closely matches the soil texture measured from the sieve–pipette method than the soil texture determined using LDPSA (RMSE = 11.88%). However, the NMR estimate of soil texture breaks down for soils with high magnetic susceptibility (>4 × 10−4 SI). These results suggest that the NMR method can provide reasonable estimates of the soil texture for soils with low magnetic susceptibility.Highlights No universal cut‐off time exists for estimating the clay, silt and sand fraction based on the NMR T2 distributions. NMR can provide better estimates of clay, silt and sand fractions for soils with low magnetic susceptibility than LDPSA. Site‐specific cut‐off times are determined through comparison between NMR data and sieve–pipette measurements. NMR yields reasonable estimates of the soil texture for soils with low magnetic susceptibility.